Method of making temperature-immune self-referencing Fabry-Pérot cavity sensors

ABSTRACT

A method of making passive microscopic Fabry-Pérot Interferometer (FPI) sensor includes forming a three-dimensional microscopic optical structure on a cleaved tip of an optical fiber that reflects a light signal back through the optical fiber. The reflected light is altered by refractive index changes in the three-dimensional structure that is subject to at least one of: (i) thermal radiation; and (ii) volatile organic compounds.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 120 asa continuation to U.S. patent application Ser. No. 16/785,718 filed on10 Feb. 2020 and entitled “Method of making temperature-immuneself-referencing Fabry-Pérot cavity sensors,” which in turn claims thebenefit of priority under 35 U.S.C. § 119(e) to U.S. ProvisionalApplication Ser. No. 62/804,996 entitled “Temperature-immuneself-referencing Fabry-Pérot cavity sensors,” filed 13 Feb. 2019, thecontents of which are incorporated herein by reference in theirentirety. This application claims the benefit of priority under 35U.S.C. § 119(e) to U.S. Provisional Application Ser. No. 62/964,210entitled “Temperature-immune self-referencing Fabry-Pérot cavitysensors,” filed 22 Jan. 2020, the contents of which are incorporatedherein by reference in their entirety.

This application is related to another application filed on the same dayunder 35 U.S.C. § 120 as a second continuation to U.S. patentapplication Ser. No. 16/785,718 and is entitled “Temperature-immuneself-referencing Fabry-Pérot cavity sensors”.

ORIGIN OF THE INVENTION

The invention described herein was made by employees of the UnitedStates Government and may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefore.

BACKGROUND 1. Technical Field

The present disclosure generally relates to optical sensors and methodsof fabricating optical sensors.

2. Description of the Related Art

The Fabry-Pérot (FP) cavity is an important optical component with manyapplications. A basic FP cavity consists of two parallel reflectivesurfaces separated by a chosen distance and encapsulating air, vacuum,or another media with refractive index (RI) n. Multiple beaminterference between the two surfaces causes transmission through thecavity to peak at specific wavelengths of maximum coherent interference,while others are reflected. At the micron scale, this enables the FPcavity to propagate a small number of optical modes compared to otheroptical cavities such as ring resonators, photonic crystals, anddistributed feedback gratings. [1] The FP cavity can also achieve largequality factors, with values as high as 105 reported. [2] It is easilyaccessible to the environment and, unlike devices such as the ringresonator, the FP cavity does not require the substance inside thecavity to have a different RI than the substance outside the cavity. [3]While often beneficial, the open nature of the FP cavity means it lackslateral confinement, and loses some resonant light off the edges of themirrors. Flat FP cavities are highly sensitive to misalignment, and anymisalignment, even one of several degrees, between the mirrors willsignificantly lower a cavity's quality factor. [1] One popular way toovercome this sensitivity is by using one or more curved mirrors, [3-5]although this often increases the complexity of fabrication.

Many advantages of the FP cavity have made it a key component to amyriad of applications. When used to form a laser cavity, a variety ofexotic gain media have recently been explored including biologicaltissues, [6] silicon nanowires, [7] and optical fluids. [1, 3]Miniaturized tunable lasers [8, 9] and tunable optical filters [10] havealso been realized by integrating an FP cavity withmicroelectromechanical systems (MEMS). The accessibility of the cavityhas also made it a powerful tool for spectroscopy. It has been used inon-chip microfluidics, [11] human breath analysis, [12] interrogation ofliving cells, [13] and compact imaging spectrometers. [14] The FP cavityis also set to play a key role in the emerging field of quantumcomputing, with cavity quantum electrodynamics (CQED) at the forefrontof many advances. It has been demonstrated in a photon emission source,[15, 4] in strong coupling to a trapped atom, [16] and in frequencysplitting of polarization eigenmodes. [17, 18]

The difference between two resonant wavelengths in a FP cavity, thecavity's free spectral range (FSR), is determined by the distancebetween the mirrors and the refractive index of the medium inside thecavity. Sensors can detect phenomena that affect these factors, and havefound many applications to include sensing gravitational waves, [19]acceleration, [20] pressure, [21] liquid RI, [21] temperature, [22]force [24], and even gas composition. [24]

Optical fibers present a powerful platform to both form and interrogateFP cavities due to their small form factor, low-loss, and well-behavedtransverse optical mode structure. Promising applications for fiberintegrated FP cavities include optofluidic in-fiber lasers [25, 26] andminiaturized high sensitivity sensors [21-24]. Poor lateral confinementand misalignment sensitivity continue to plague fiber based FP cavitydevices, and represent significant design challenges. The fiber itselfis also an exotic substrate due to its geometry, which renders itincompatible with many planar microfabrication processes. A variety oftechniques have been explored to overcome these challenges and create FPcavities on optical fibers. One device was fabricated by splicing asegment of hollow-core optical fiber (HOF) to a single-mode fiber (SMF),and capping the HOF with a segment of photonic crystal fiber (PFC). [24]While this design can interrogate gasses, liquids would have difficultyreaching the cavity through the small openings in the PFC. Splicingvarious types of optical fibers also requires precise alignment and maybe difficult to repeat reliably. Another successful on-fiber FPresonator was made by ion milling a cavity into a tapered SMF probe.[22] While the environment is easily accessed by this cavity, thefabrication process is complex and laborious, involving CO₂ laserpulling and metal deposition before the ion milling. Another group usedthe photo-active polymer SU-8 to construct a suspended polymer cavity ona fiber tip. [21] The resulting device can interrogate liquid or gas,and the fabrication process enables two-dimensional (2-D) freedom with adigital mirror. But this process is also relatively complicated,requiring a spray coat, bake, and UV exposure for each individual layer.This also limits the 3-D structures that can be realistically built.

BRIEF DESCRIPTION OF THE DRAWINGS

The description of the illustrative embodiments can be read inconjunction with the accompanying figures. It will be appreciated thatfor simplicity and clarity of illustration, elements illustrated in thefigures have not necessarily been drawn to scale. For example, thedimensions of some of the elements are exaggerated relative to otherelements. Embodiments incorporating teachings of the present disclosureare shown and described with respect to the figures presented herein, inwhich:

FIG. 1A is a top isometric projection of a single-cavity FP sensor on acleaved end of an optical fiber, according to one or more embodiments;

FIG. 1B is a side view of the single-cavity FP sensor on a cleaved endof the optical fiber of FIG. 1A, according to one or more embodiments;

FIG. 1C is a top isometric projection of a released dual-cavity FPsensor on a cleaved end of an optical fiber, according to one or moreembodiments;

FIG. 1D is a side view of the released dual-cavity FP sensor on acleaved end of the optical fiber of FIG. 1C, according to one or moreembodiments;

FIG. 1E is a top isometric projection of a released hemisphericaldual-cavity FP sensor on a cleaved end of an optical fiber, according toone or more embodiments;

FIG. 1F is a side view of the released hemispherical dual-cavity FPsensor on a cleaved end of the optical fiber of FIG. 1E, according toone or more embodiments;

FIG. 2A depicts a top isometric projection of a fiber tip that wasproperly cleaned, cleaved, and mounted on a laser machining station,according to one or more embodiments;

FIG. 2B depicts a top isometric projection of photosensitive resin thatwas deposited on the fiber tip of FIG. 2A, according to one or moreembodiments;

FIG. 2C depicts a top isometric projection of a femtosecond laser thatwas then focused in the photosensitive IP-DIP resin of FIG. 2B topolymerize portions of resin layer by layer to form a solidifiedstructure, according to one or more embodiments;

FIG. 2D depicts a top isometric projection of the fiber tip after achemical developer was used to remove non-polymerized resin, releasingthe solidified structure, according to one or more embodiments;

FIG. 3A depicts a top view of an AFM of the surface interrogated,according to one or more embodiments;

FIG. 3B depicts a 3-D rendering of the AFM scan of FIG. 3A showingsurface topography, according to one or more embodiments;

FIG. 3C depicts three cross sections I-III throughout the surface ofFIG. 3A to quantify roughness, according to one or more embodiments;

FIG. 4 depicts a diagram of an experimental setup used to characterizethe reflection spectrum of each device in air at room temperature,according to one or more embodiments;

FIG. 5A depicts a scanning electron microscope (SEM) image of afabricated single-cavity FP device, according to one or moreembodiments;

FIG. 5B depicts a graphical plot of measured reflection intensity as afunction of wavelength for the fabricated single-cavity FP device ofFIG. 5A, according to one or more embodiments;

FIG. 5C depicts a graphical plot of an Airy distribution as a functionof wavelength for the fabricated single-cavity FP device of FIG. 5A,according to one or more embodiments;

FIG. 6A depicts a SEM image of a fabricated released dual-cavity FPdevice, according to one or more embodiments;

FIG. 6B depicts a graphical plot of measured reflection intensity as afunction of wavelength for the released dual-cavity FP device of FIG.6A, according to one or more embodiments;

FIG. 6C depicts a graphical plot of an Airy distribution as a functionof wavelength for the released dual-cavity FP device of FIG. 6A,according to one or more embodiments;

FIG. 7A depicts a SEM image of a fabricated hemispherical dual-cavity FPdevice, according to one or more embodiments;

FIG. 7B depicts a graphical plot of measured reflection intensity as afunction of wavelength for the hemispherical dual-cavity FP device ofFIG. 7A, according to one or more embodiments;

FIG. 7C depicts a graphical plot of an Airy distribution as a functionof wavelength for the hemispherical dual-cavity FP device of FIG. 7A,according to one or more embodiments; and

FIG. 8 depicts a flow diagram of a method for fabricating a passiveoptical sensor on a tip of an optical fiber, according to one or moreembodiments.

DETAILED DESCRIPTION

According to aspects of the present disclosure, a passive microscopicFabry-Pérot Interferometer (FPI) sensor includes an optical fiber athree-dimensional microscopic optical structure formed on a cleaved tipof an optical fighter that reflects a light signal back through theoptical fiber. The reflected light is altered by refractive indexchanges in the three-dimensional structure that is subject to at leastone of: (i) thermal radiation; and (ii) volatile organic compounds.

According to aspects of the present disclosure, a method is provided forfabricating a passive optical sensor on a tip of an optical fiber. Inone or more embodiments, the method includes perpendicularly cleaving atip of an optical fiber. The method includes mounting the tip of theoptical fiber in a specimen holder of a photosensitive polymerthree-dimensional micromachining machine. The method includes forming athree-dimensional microscopic optical structure on the tip that reflectsa light signal back through the optical fiber. The reflected light isaltered by refractive index changes in the three-dimensional structurethat is subject to at least one of: (i) thermal radiation; and (ii)volatile organic compounds.

The present disclosure introduces an innovative fabrication process thatgreatly simplifies the realization of complex geometries on virtuallyany substrate. Three-dimensional (3-D) Fabry-Pérot cavities are sculptedon fiber tips using a multiphoton polymerization process. In particular,3-D Fabry-Pérot (FP) cavities are fabricated directly onto cleaved endsof low-loss optical fibers by a two-photon polymerization process. Thisfabrication technique is quick, simple, and inexpensive compared toplanar microfabrication processes, which enables rapid prototyping andthe ability to adapt to new requirements. These devices also utilizetrue 3-D design freedom, facilitating the realization of microscaleoptical elements with challenging geometries. Three different devicetypes were fabricated and evaluated: an unreleased single-cavity device,a released dual-cavity device, and a released hemispherical mirrordual-cavity device. Each iteration improved the quality of the FPcavity's reflection spectrum. The unreleased device demonstrated anextinction ratio around 1.90, the released device achieved 61, and thehemispherical device achieved 253, providing a strong signal to observechanges in the free spectral range (FSR) of the device's reflectionresponse. The reflectance of the photopolymer was also estimated to bebetween 0.2 and 0.3 over the spectrum of interest. The dual-cavitydevices include both an open cavity, which can interact with aninterstitial medium, and a second solid cavity, which provides a staticreference reflection. The hemispherical dual-cavity device furtherimproves the quality of the reflection signal with a more consistentresonance, and reduced sensitivity to misalignment. These advancedfeatures, which are very challenging to realize with traditional planarmicrofabrication techniques, are fabricated in a single patterning step.

FIG. 1A is a top isometric projection of a single-cavity FP sensor 100 aon a cleaved end (fiber tip 104) of an optical fiber 102. FIG. 1B is aside view of the single-cavity FP sensor 100 a on the cleaved end (fibertip 104) of the optical fiber 102. FIG. 1C is a top isometric projectionof a released dual-cavity FP sensor 100 b on a cleaved end (fiber tip104) of an optical fiber 102. FIG. 1D is a side view of the releaseddual-cavity FP sensor 100 b on a cleaved end (fiber tip 104) of theoptical fiber 102. FIG. 1E is a top isometric projection of a releasedhemispherical dual-cavity FP sensor 100 c on a cleaved end (fiber tip104) of an optical fiber 102. FIG. 1F is a side view of the releasedhemispherical dual-cavity FP sensor 100 c on a cleaved end (fiber tip104) of the optical fiber 102.

The devices presented in this work were fabricated using a simpleprocess that requires only mounting the fiber into a two-photonpolymerization system from Nanoscribe GmbH and chemical developing [27].This technique enabled us to realize 3-D free-form geometries—a featwhich cannot be accomplished using other methods on this spatial scale.This enables the use of nonplanar components to improve deviceperformance, such as in our use of curved mirrors to create ahemispherical FP cavity to significantly reduce misalignmentsusceptibility. Our method can create these 3-D components withsubmicron precision. The three on-fiber FP cavity designs that werefabricated and tested are depicted in FIGS. 1A-IF. The hemisphericaldevice achieved the greatest extinction ratio of the three designstested, and highlights the power of the design freedom afforded by thisprocess. The unreleased device demonstrated an extinction ratio around1.90, the released device achieved 61, and the hemispherical deviceachieved 253, providing a strong signal to observe changes in the FSR ofthe device. We were also able to extract the reflectance of thephotopolymer by fitting an Airy distribution to the reflection spectrum.This yielded a reflectance between 0.2 and 0.3 for the polymerizedresin. The dual-cavity devices allow for interrogation of aninterstitial medium in the first, open cavity while simultaneouslyreferencing the static reflection spectrum of the second, solid-polymercavity. These advanced features, which are very difficult or impossibleto achieve with traditional planar microfabrication techniques, werefabricated in a single patterning step. The speed and simplicity offabrication enables rapid prototyping and an iterative design processesto realize complicated devices and advanced features.

Fabrication Process: FIGS. 2A-2D depict a sequential example of masklesstwo-photon polymerization microfabrication process flow. In theillustrative embodiments, devices fabricated in this work were centeredon the fiber core 108. FIG. 2A depicts a top isometric projection of afiber tip 104 of an optical fiber 102 that was properly cleaned,cleaved, and mounted on a laser machining station. First the opticalfiber was stripped, cleaned, and cleaved to create a flat platform withaccess to the core of the fiber, as illustrated in FIG. 2A. The opticalfiber 102 used in this work was F-SM1500-9/125-P fiber from NewportCorporation. The cleaved optical fiber 102 was secured into a NewportFPH-S fiber chuck and mounted into a custom jig that aligns a cleavedfiber face 108 orthogonally to the laser aperture of the Nanoscribe GmbHsystem.

FIG. 2B depicts a top isometric projection of photosensitive IP-DIPresin 110 (marketed by Nanoscribe GmbH) that was deposited on the fibertip 104. The uncured, liquid photoactive polymer resin (Nanoscribe'sIP-DIP) was deposited onto the cleaved fiber face, as shown in FIG. 2B.The resin can be deposited in any thickness or shape that encloses thedesired build volume. Thus, several traditional photoresist depositionsteps, such as spin-coating and pre-baking, were eliminated. FIG. 2Cdepicts a top isometric projection of a femtosecond laser 112 that wasthen focused in the photosensitive IP-DIP resin 110 of FIG. 2B topolymerize portions of resin 110 layer by layer to form a solidifiedstructure 114. Once mounted, the resin 110 was selectively exposed toultra-short laser pulses with a wavelength of 780 nm, a repetition rateof 80 MHz, and a pulse duration of 120 fs by the Nanoscribe GmbH laserwriting system. The resin 110 only solidified when subjected to thenonlinear optical process of two-photon polymerization. Simultaneousabsorption of two photons was necessary to polymerize the resin, whichonly occurred in a small portion of the focused laser beam. [28] Thevolume of the beam initiating two-photon polymerization can be scaled tooffer a balance of resolution and speed. The minimum volume for thissystem was 150 nm wide by 150 nm long by 200 nm tall. This focal pointwas scanned through the resin according to a computer-aided design (CAD)file to solidify the desired structure. This system has a maximum scanspeed of 2 mm/s, but small features and optical quality curvaturesrequire significantly slower scan speeds. The FP cavity devices in thiswork were fully polymerized in less than 15 minutes. The x-y aspects ofeach layer were controlled by a galvanometer, while the z direction wascontrolled with a piezoelectric actuator.

FIG. 2D depicts a top isometric projection of the fiber tip 104 after achemical developer was used to remove non-polymerized resin 110 (FIG.2C), releasing the solidified structure 114. In particular, once thedesired volume had been polymerized, the fiber tip 104 was submerged inpropylene glycol methyl ether acetate (PGMEA) for 20 min. This commonsolvent removed the unexposed resin, releasing the polymerizedstructure. Finally, the fiber tip 104 was submerged in isopropanol (IPA)for another 20 min to clean off the PGMEA. The result was the desired3-D structure (single-cavity FP sensor 100 a) of polymerized resin.

While significantly faster than other fabrication methods, the stepwisenature of the laser scanning process introduced striations into thesurface finish of the devices. Planar FP cavities require a flatreflective surface, and hemispherical FP cavities require a smoothspherical mirror, and it was not known if the devices created here hadan optical-quality surface finish. Also of concern, features with aheight equal to one half or one quarter of the wavelength of interestcould introduce destructive interference and create an antireflectivesurface. To analyze the surface finish, we fabricated a sample structureonto an Indium-Tin-Oxide (ITO) coated glass slide to mount into anatomic force microscope (AFM).

FIGS. 3A-3C depict surface analysis of an optical flat 300 fabricatedwith two-photon polymerization by ultra-short laser pulses. FIG. 3Adepicts a top view of an AFM of the surface interrogated. FIG. 3Bdepicts a 3-D rendering 320 of the AFM scan of FIG. 3A showing surfacetopography. FIG. 3C depicts three cross sections I-III 340 a-340 cthroughout the surface of FIG. 3A to quantify roughness. The expectedstriations from the scanning process were present at regular intervals.The surface finish, including these features, was found to have aroughness of approximately 60 nm, with the peak to peak differenceaveraging 120 nm. This work focused on using wavelengths in the1460-1640 nm range to probe the FP structures fabricated on the fiberends. Therefore, the fabrication variations in surface roughness aresignificantly smaller than the wavelengths of interest, and far lessthan one half or one quarter wavelength interval which would lead totheir own interference effects. In fact, the structures fabricated bythis process were confirmed to have a roughness below λ/10 which isconsistent with an optical quality surface finish in the wavelengths ofinterest.

FIG. 4 depicts a diagram of an experimental setup 400 used tocharacterize the reflection spectrum of each device in air at roomtemperature. Experimental setup 400 includes tunable laser source 402(Agilent 81600B) that outputs a laser source via laser input fiber 403to a first port 404 of optical circulator 406. Tunable laser source 402was swept from 1463 nm to 1634 nm during each measurement. The lasersource is directed from second port 707 of optical circulator 406through SM optical fiber (“device fiber”) 408 to fiber tip device 410and exposed to atmosphere 412. In particular, device fiber 408 wasfusion spliced to another SMF terminating in an FC/APC connector using aFujikura FSM-100P ARC Master fiber splicer. Reflections from fiber tipdevice 410 return through SM optical fiber 408 to second port 407 ofoptical circulator 406 to third port 414 of optical circulator 406 fordetection by photo detector 416. Power meter 418 measures results fromphoto detector 416 for presenting on digital oscilloscope 420. Inparticular, the third port 414 of the optical circulator 406 wasconnected to a Newport universal fiber optic detector. Thisphotodetector interfaced with a Newport 1830-C optical power meter,whose output was visualized and stored using a Keysight D509254A digitalstorage oscilloscope.

The optical circulator 406 and laser input fiber 403 were polarizationmaintaining, while the device fiber 408 was not. Therefore thepolarization was adjusted to maximize reflected intensity at 1550 nm atthe start of each measurement. The tunable laser source 402 was thenswept from 1463 nm to 1634 nm and the reflection from the FP cavity 422of the fiber tip device 410 was isolated by the optical circulator 406.The photodetector and power meter transduced the optical power into avoltage which was monitored and recorded by the oscilloscope. Opticalresonances within the FP cavity caused a peak in transmission throughthe FP cavity which was observed as a dip in reflection intensity. Thistechnique allowed the devices to be used remotely, with the bulky inputand measurement components geographically separated from the device.

The reflection spectrum of each device was measured in volts read by theoscilloscope at a given wavelength. The extinction ratio reported foreach device was calculated using the ratio between the mean of the fourlowest reflection dips and the mean of the four highest reflectionpeaks. The mean of the four highest peaks is referred to as the highreflection intensity, and the mean of the four lowest dips is referredto as the low reflection intensity. Assuming incident light normal toeach cavity, the theoretical FSR of a FP cavity is calculated accordingto Δλ_(FSR)=λ₀ ²/2nl, where λ₀ is the central wavelength of thetransmission peak (and reflection dip), n is the RI of the cavitymedium, and nn is the length of the cavity. For our devices, weconsidered a hypothetical transmission peak at 1550 nm, an IP-DIPrefractive index of 1.504, and a refractive index of 1 in air. The RIwas calculated by interpolating data provided by Nanoscribe, and 1550 nmis a common telecom wavelength in the middle of our laser's bandwidth.All calculations assume room temperature.

The transmission through a FP resonator can be modelled by the Airydistribution, which calculates the internal resonance enhancement factorfor light of a given wavelength based on the physical properties of thecavity. [29] The generic Airy distribution for two mirrors of equalreflectance is calculated with, A=[(1−R)²+4R sin²(ϕ)]⁻¹, where R is thereflectance of the mirrors, and 2ϕ is the single-pass phase shiftbetween the mirrors. [29] This is calculated with,2ϕ′=2π(λ−λ₀)/Δλ_(FSR)≈2πλ/Δλ_(FSR). The intensity of light reflectedback from the cavity, as was measured in this work, is inverselyproportional to the transmission intensity.

We extracted the reflectance of the mirrors in our devices by fitting anAiry distribution to the measured reflection spectrum. To create acomparable waveform, we selected the FSR and first λ₀ from ourmeasurements, and centered the phase shift at the initial resonantwavelength by subtracting it from λλ to determine the single-pass phaseshift in relation to the resonant wavelength, 2ϕ′=2π(λ−λ₀)/Δλ_(FSR). Thedistribution was also normalized and scaled to the maximum and minimumvoltage readings for each device. For the dual-cavity devices, the FSRand initial resonant wavelengths of each cavity were used to calculatetwo Airy distributions, which were added together, then normalized andscaled to the magnitude of the measured reflection. This showed theideal response of each device given the measured FSR, resonantwavelength, and magnitude. With this waveform, different values of Rwere chosen until the magnitude and shape closely resembled the measuredresponse. The value that provided the best match was taken as thereflectance.

Measurement Results: FIG. 5A depicts a scanning electron microscope(SEM) image of a fabricated single-cavity FP device 500 comprising a 40μm by 40 μm and 17.58 μm tall rectangle on a cleaved tip of an opticalfiber 502. FIG. 5B depicts a graphical plot 530 of measured reflectionintensity as a function of wavelength. The cavity was formed by a 17.58μm long, 40 μm by 40 μm rectangle, resulting in a theoretical FSR of45.43 nm. The measured average Δλ_(FSR) was 42.09 nm, showing avariation of only 3.34 nm. The device's high reflection intensity was19.29 μW corresponding to a voltage of 132.58 mV. The low reflectionintensity was 10.17 with a voltage of 69.89 mV, yielding an extinctionratio of 1.90. FIG. 5C depicts a graphical plot 560 of an Airydistribution with R=0.01 as a function of wavelength. Fitting the Airydistribution to these results gave a reflectance of 0.01. This low valuewas caused by the thicker fiber-polymer interface, as the releaseddevices show significantly higher reflectance. The single-cavity deviceconfirmed that the two-photon polymerization method successfullyproduced optical elements for planar FP resonators.

FIG. 6A depicts a SEM image of a fabricated dual-cavity FP device 600formed on a cleaved end of SMF 602. Dual-cavity FP device 600 has a 56μm diameter, 20 μm tall disk suspended above a 35 μm air cavity. Thesecavity lengths match the hemispherical device. (b) Measured reflectionintensity as a function of wavelength. (c) Airy distribution with R=0.3.The released dual-cavity device represents a significant improvement infunctionality over the single-cavity device because its first cavity isopen to the environment. FIG. 6B depicts a graphical plot 630 ofmeasured reflection intensity as a function of wavelength. Thedual-cavity FP device also improved the extinction ratio of thereflection spectrum. The first cavity was 35 μm tall and filled withair, leading to a theoretical FSR of 34.32 nm. The polymer cavity wasformed by a 56 μm diameter, 20 μm tall disk with a theoretical FSR of39.94 nm. When measured, the air cavity had an average FSR of 36.24 nmand the polymer cavity had an average FSR of 36.07, agreeing within fivenanometers of the theoretical values. The high reflected intensity ofthis device was 68.24 corresponding to a voltage of 468.98 mV, and thelow reflected intensity was 1.12 μW reading a voltage of 7.67 mV. Thisgives the device an extinction ratio of 61. FIG. 6C depicts a graphicalplot 660 of an Airy distribution as a function of wavelength withreflectance R=0.03.

Suspending the polymer structure over an air-gap allows variousinterstitial media to be introduced into the first cavity. Optofluidicdies, quantum dot suspensions, or other gain media could be inserted tocreate fiber-tip lasers. The RI of an unknown gas or liquid can also bedetermined by immersing the dual-cavity device and comparing the shiftedFSR to a reference. Furthermore, by including both a solid polymercavity and an open cavity, an RI sensor with this device would beself-referencing and temperature immune. If a single open cavity sensorwas exposed to both a change in temperature and interstitial medium, theFSR of the device would shift due to the new RI of the cavity, and thenew cavity length introduced by thermal expansion of the polymer. Itwould be very difficult to decouple each effect from the observed FSRshift. The released dual-cavity device would be able to isolate a changein RI from the effect of thermal expansion because the solid cavitywould only experience the thermal effects. One could determine thethermal effects from the FSR shift in the polymer cavity, calculate thecorresponding effects on the open cavity, and subtracting them from theopen cavity FSR shift to isolate the changes in the interstitial medium.

While the dual-cavity device enables many applications, there was a riskthat the response from one cavity would interfere with the response fromanother. If the resonant wavelengths are too close and the width of theresonance is too large, different peaks could not be resolved.Furthermore, light reflected from one cavity could destructivelyinterfere with light resonating in another, removing part of the signal.Fortunately, this kind of interference can be avoided by properlydesigning the constituent optical cavities. Peaks from both cavities areclearly resolvable, as seen in FIG. 6B, and the extinction ratioimproved markedly over the single-cavity device. Since the signalquality improved with the addition of the second cavity, interferencebetween the cavities does not seem to degrade the response.

FIG. 7A depicts a SEM image of a fabricated hemispherical dual-cavity FPdevice 700 having a 56 μm diameter, 20 μm tall cylinder measured fromeach center of curvature. The top mirror has a 75 μm radius and theinner mirror has a 35 μm radius. The center of the inner mirror issuspended 22.5 μm above the face of the fiber. FIG. 7B depicts agraphical plot 730 of measured reflection intensity as a function ofwavelength for the hemispherical dual-cavity FP device 700 of FIG. 7A.FIG. 7C depicts a graphical plot 760 of an Airy distribution as afunction of wavelength with reflectance R=0.02 for the hemisphericaldual-cavity FP device 700 of FIG. 7A.

The hemispherical device enjoys all the utility of the flat dual-cavitydevice while adding the many benefits of curved mirrors. Thehemispherical mirrors reduced diffraction losses and improved lateralconfinement to produce a more consistent peak transmission. Within thereflection dips, the flat dual-cavity device showed a variance of 0.53mV, while the hemispherical counterpart achieved a variance of only0.026 mV. The hemispherical FP cavity also had the largest extinctionratio observed, with a high reflection intensity of 78.162 μW reading537.17 mV, and a low reflection intensity of 0.31 μW reading 2.12 mV.This gave the hemispherical device an extinction ratio of 253. An SEMimage, and the reflection spectrum of the hemispherical Fabry-Pérotcavity are shown in FIG. 7A and FIG. 7B respectively. The distancebetween the center of the inner mirror and the face of the fiber was 35μm, with a theoretical FSR of 34.32 nm. Within the polymer gap, theconcave-convex resonator was 20 μm long between centers for atheoretical FSR of 39.94 nm. The FSR of the air cavity was measured tobe 36.31 nm, and the FSR of the polymer cavity was measured to be 36.19nm. The Airy distribution fit the measured response with a reflectanceof 0.2. This value is lower than the flat cavity, although the curvedmirror reduces losses. The drop is likely caused by the thin polymerfeature fabricated over the surface of the fiber. While the feature doesnot function as intended, as a third curved mirror, the fiber-polymerinterface it creates explains the loss in reflectance.

Like all curved-mirror FP cavities, hemispherical resonators aresignificantly less sensitive to misalignment, making the device morerobust in the face of vibrations or impacts to the fiber. In addition,they can be used at higher incident intensities without the loss ofresolution that occurs in planar FP cavities. The hemispherical devicerepresents the power of our fabrication technique to utilize 3-D freedomto create advantageous geometries that cannot otherwise be realized.

In conclusion, we have demonstrated three FP resonator designsfabricated directly onto the cleaved ends of low-loss optical fibers.Our fabrication technique is simple, fast, and enables true 3-D freedomto realize complex features, such as optical elements, which aredifficult or impossible to create with traditional microfabricationmethods. Two-photon polymerization with ultra-fast laser pulses createsdevices on fiber tips in less than 15 minutes with a single writingstep. Each device improved the quality of the FP cavity's reflectionresponse. The single-cavity featured an extinction ratio of 1.90, thereleased dual planar cavity device obtained an extinction ratio of 61,and the hemispherical cavity device obtained an extinction ratio of 253.The reflectance of the direct fiber-polymer interface was estimated tobe 0.01, while the reflectance of IP-DIP was estimated to be between 0.2and 0.3, both over 1463 nm to 1634 nm. The dual-cavity device promisesincreased utility as the open cavity is able to interact with itsenvironment and reference changes in RI to the solid cavity. Thehemispherical device brings the benefits of a curved-mirror FP resonatorsuch as improved alignment insensitivity and constant resolution atincreased intensity, while also providing a consistent resonantintensity across the spectrum of interest.

These FP cavity devices invite numerous applications, such as on-fiberlasers and various sensors. Our future work specifically hopes toexplore reflective coatings to improve the reflectance and qualityfactor of the reflection spectrum. Micron scale plano and hemisphericalcavities with have demonstrated quality factors as high as 105 by addinga reflective coating, [2] and the next devices we are producing aim toachieve a reflectance of 0.9 or higher. The speed of our fabricationprocess enables us to take an iterative design approach and exploreseveral reflective coating options. A more reflective FP cavity willhave much higher resolution and produce measurable responses for smallchanges in the RI or length of the cavity to detect different phenomenasuch as electromagnetic radiations, acoustic waves, temperature andpressure changes, displacements, and hazardous material concentrationsin both gas and liquid form. Future work includes determining thesuitability of the next generation devices to sense some of thesephenomena.

FIG. 8 depicts a flow diagram of a method 800 for fabricating a passiveoptical sensor on a tip of an optical fiber. Method 800 includesproviding an optical fiber that supports single mode light propagationfor a 1550 to 1650 nm wavelength range and having a polyimide sheathingwhich can withstand temperatures up to 300° C. (block 802). Method 800includes perpendicularly cleaving a tip of the optical fiber (block804). Method 800 includes depositing a photosensitive polymer on thecleaved tip (block 806). Method 800 includes mounting the tip of theoptical fiber in a specimen holder of a photosensitive polymerthree-dimensional micromachining machine (block 808). In one or moreembodiments, method 800 includes mounting the tip of the optical fiberin the specimen holder by attaching a fiber chuck over a semiconductorwafer opening in a specimen tray. Method 800 includes forming athree-dimensional microscopic optical structure within thephotosensitive polymer that comprises a Fabry-Pérot Interferometer (FPI)using the three-dimensional micromachining machine that performs twophoton process (block 810). Method 800 includes removing an uncuredportion of the photosensitive polymer using a solvent (block 812). Inone or more embodiments, method 800 includes forming thethree-dimensional microscopic optical structure by forming a layer ofpolymer that acts as a single cavity FPI that reflects a light signalback through the optical fiber, the reflected light altered byrefractive index changes in the three-dimensional structure that issubject to at least one of: (i) thermal radiation; and (ii) volatileorganic compounds. In one or more embodiments, method 800 includesforming the three-dimensional microscopic optical structure by forming asuspended layer of polymer providing three flat interfaces that act as atwo cavity FPI. In one or more embodiments, method 800 includes formingthe three-dimensional microscopic optical structure by forming asuspended layer of polymer having double confocal surfaces that act as atwo cavity FPI. Then method 800 ends.

The following references cited above are hereby incorporated byreference in their entirety:

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While the disclosure has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the disclosure. Inaddition, many modifications may be made to adapt a particular system,device or component thereof to the teachings of the disclosure withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the disclosure not be limited to the particular embodimentsdisclosed for carrying out this disclosure, but that the disclosure willinclude all embodiments falling within the scope of the appended claims.Moreover, the use of the terms first, second, etc. do not denote anyorder or importance, but rather the terms first, second, etc. are usedto distinguish one element from another.

In the preceding detailed description of exemplary embodiments of thedisclosure, specific exemplary embodiments in which the disclosure maybe practiced are described in sufficient detail to enable those skilledin the art to practice the disclosed embodiments. For example, specificdetails such as specific method orders, structures, elements, andconnections have been presented herein. However, it is to be understoodthat the specific details presented need not be utilized to practiceembodiments of the present disclosure. It is also to be understood thatother embodiments may be utilized and that logical, architectural,programmatic, mechanical, electrical and other changes may be madewithout departing from general scope of the disclosure. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present disclosure is defined by the appendedclaims and equivalents thereof.

References within the specification to “one embodiment,” “anembodiment,” “embodiments”, or “one or more embodiments” are intended toindicate that a particular feature, structure, or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present disclosure. The appearance of such phrases invarious places within the specification are not necessarily allreferring to the same embodiment, nor are separate or alternativeembodiments mutually exclusive of other embodiments. Further, variousfeatures are described which may be exhibited by some embodiments andnot by others. Similarly, various requirements are described which maybe requirements for some embodiments but not other embodiments.

It is understood that the use of specific component, device and/orparameter names and/or corresponding acronyms thereof, such as those ofthe executing utility, logic, and/or firmware described herein, are forexample only and not meant to imply any limitations on the describedembodiments. The embodiments may thus be described with differentnomenclature and/or terminology utilized to describe the components,devices, parameters, methods and/or functions herein, withoutlimitation. References to any specific protocol or proprietary name indescribing one or more elements, features or concepts of the embodimentsare provided solely as examples of one implementation, and suchreferences do not limit the extension of the claimed embodiments toembodiments in which different element, feature, protocol, or conceptnames are utilized. Thus, each term utilized herein is to be given itsbroadest interpretation given the context in which that terms isutilized.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The description of the present disclosure has been presented forpurposes of illustration and description, but is not intended to beexhaustive or limited to the disclosure in the form disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope of the disclosure. Thedescribed embodiments were chosen and described in order to best explainthe principles of the disclosure and the practical application, and toenable others of ordinary skill in the art to understand the disclosurefor various embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A method for fabricating a passive optical sensoron a tip of an optical fiber, the method comprising: perpendicularlycleaving a tip of an optical fiber; mounting the tip of the opticalfiber in a specimen holder of a photosensitive polymer three-dimensionalmicromachining machine; and forming a three-dimensional microscopicoptical structure on the tip that reflects a light signal back throughthe optical fiber, the reflected light altered by refractive indexchanges in the three-dimensional structure that is subject to at leastone of: (i) thermal radiation; and (ii) volatile organic compounds,wherein the three-dimensional microscopic optical structure comprises asuspended layer of polymer providing three flat interfaces that act as atwo cavity FPI.
 2. The method of claim 1, wherein forming thethree-dimensional microscopic optical structure comprises performing atwo-photon polymerization process of a photosensitive polymer using athree-dimensional micromachining device.
 3. The method of claim 1,wherein perpendicularly cleaving the tip of the optical fiber comprisesproviding the optical fiber that supports single mode light propagationfor a 1550 to 1650 nm wavelength range and having a polyimide sheathingwhich can withstand temperatures up to 300° C.
 4. The method of claim 1,wherein mounting the tip of the optical fiber in the specimen holdercomprises attaching a fiber chuck over a semiconductor wafer opening ina specimen tray.
 5. A method for fabricating a passive optical sensor ona tip of an optical fiber, the method comprising: perpendicularlycleaving a tip of an optical fiber; mounting the tip of the opticalfiber in a specimen holder of a photosensitive polymer three-dimensionalmicromachining machine; and forming a three-dimensional microscopicoptical structure on the tip that reflects a light signal back throughthe optical fiber, the reflected light altered by refractive indexchanges in the three-dimensional structure that is subject to at leastone of: (i) thermal radiation; and (ii) volatile organic compounds,wherein the three-dimensional microscopic optical structure comprises asuspended layer of polymer having double confocal surfaces that act as atwo cavity FPI.
 6. The method of claim 5, wherein forming thethree-dimensional microscopic optical structure comprises performing atwo-photon polymerization process of a photosensitive polymer using athree-dimensional micromachining device.
 7. The method of claim 5,wherein perpendicularly cleaving the tip of the optical fiber comprisesproviding the optical fiber that supports single mode light propagationfor a 1550 to 1650 nm wavelength range and having a polyimide sheathingwhich can withstand temperatures up to 300° C.
 8. The method of claim 5,wherein mounting the tip of the optical fiber in the specimen holdercomprises attaching a fiber chuck over a semiconductor wafer opening ina specimen tray.